1. Field of the Invention
The present invention relates to a manufacturing process for a semiconductor device, particularly relates to a process for manufacturing a semiconductor device where by ALD (Atomic Layer Deposition), a film is formed on a semiconductor substrate comprising a plurality of deep holes on its surface.
2. Description of the Related Art
In manufacturing a semiconductor device, a film deposited by CVD (Chemical Vapor Deposition) is frequently used. CVD is a procedure where a source gas is reacted under certain temperature conditions in the gas phase and generating reactant molecules thus is deposited on a semiconductor substrate surface. However, as a size of a semiconductor device has been reduced, surface irregularity has become significant, and as a distance between concaves has been reduced, it has become more difficult to form a film with an even thickness precisely according to an irregular shape by the above CVD.
For overcoming the difficulty in the CVD, ALD has been recently used. ALD is a procedure where a source gas and a reactant gas are alternately supplied to deposit a film. First, during supplying a source gas, the supplied source gas is adsorbed to a semiconductor substrate surface. Then, source gas molecules are adsorbed to cover the surface of the semiconductor substrate, resulting in termination of the adsorption reaction. After purging the remaining source gas, during supplying a reactant gas as a next step, the oxidative or reducing reactant gas is contacted with the adsorbed source gas for initiating a reaction between them to form a desired film molecule. Subsequently, the step pursing the reactant gas and the step supplying the source gas again are alternately repeated to form a film by monomolecular layer until the resulting film has a desired thickness.
In contrast to CVD, a dominant deposition mechanism in the ALD is a surface adsorption reaction, so that a film with an even thickness can be formed even when the surface has an irregular shape. Japanese Laid-open Patent Publication No. 2000-54134 has disclosed ALD using a single-wafer processing machine. However, as described above, ALD is a process where deposition of a monomolecular layer is repeated, leading to extremely low productivity. Film deposition by ALD using a single-wafer processing machine is, therefore, unsuitable as a mass production technique for a semiconductor device.
For overcoming poor productivity with the above single-wafer processing machine, Japanese Laid-open Patent Publication No. 2004-23043 has described a process for forming a film by ALD using a vertical batch processing machine with a furnace body. This processing machine can improve significantly productivity because a film can be simultaneously formed on a plurality sheets of semiconductor substrate by ALD.
FIG. 1 shows an example of a vertical batch processing machine with a furnace body. A gas supply system in the vertical batch processing machine described in FIG. 1 in Japanese Laid-open Patent Publication No. 2004-23043 comprises a similar configuration to the above vertical batch processing machine.
In the vertical batch processing machine shown in FIG. 1, the top of a reaction tube 103a constituting a reaction chamber 103 comprises a vacuum exhaust outlet, which is connected to a vacuum valve 106 via a connection unit 105 and further connected to a vacuum pump 109 via a pressure-regulating valve 107 and a vacuum line 108. In the reaction chamber 103, a boat 101 is placed such that the boat 101 is supported by a boat loader 102 and is disposed a plurality of semiconductor substrates 100. Furthermore, a heater 104 for heating the semiconductor substrate surrounds the reaction tube 103a, in contact with its outer surface.
This vertical batch processing machine comprises a supply source for trimethylaluminum (TMA: Al(CH3)3) as a metal source gas, and the TMA supply source is connected to a gas injector 114 comprising a plurality of small holes via a TMA inlet valve 110, a liquid flow regulator (LMFC) 111, a vaporizer 112 and a valve 113. The plurality of small holes in the gas injector 114 are disposed, corresponding to the positions of the plurality of semiconductor substrates 100. To the vaporizer 112, a nitrogen (N2) or argon (Ar) supply source is connected via a TMA carrier gas inlet valve 116 and a flow regulator (MFC) 115.
On the other hand, ozone (O3) gas as one of reactant gases is generated by an O3 generator (ozonizer) 118 using a gas fed from an oxygen (O2) supply source via a flow regulator (MFC) 117 and then supplied to the reaction chamber from a gas injector 122 via an O3 inlet valve 119. For purging the O3 supply line, N2 or Ar are supplied from an N2 or Ar supply source via a flow regulator (MFC) 120 and a valve 121.
Steam (H2O) to be the other reactant gas is connected to a gas injector 124 from an H2O supply source via a flow regulator (MFC) 122 and a valve 123. For purging a (H2O) supply line, N2, Ar or O2 is supplied from an N2, Ar or O2 supply source via a flow regulator (MFC) 125 and a valve 126.
FIG. 2 shows a gas flow sequence when a tantalum oxide film is deposited by ALD as illustrated in FIG. 2 in Japanese Laid-open Patent Publication No. 2004-23043. A vertical batch processing machine similar to that described above is used, and pentaethoxytantalum (PET) is used as a metal source gas, O2 is used as a reactant gas and H2O is used as a pre-treatment agent before supplying PET. In FIG. 2, the horizontal axis is an elapsed time and the vertical axis is a parallel flow sequence of gases fed from individual gas injectors. One cycle (120 sec) comprises supplying, first, H2O for 45 sec and purging the gas for 20 sec, next supplying PET for 20 sec and purging the gas for 10 sec, and finally supplying O2 for 20 sec and purging the gas for 5 sec. In each gas purging, Ar fed from each gas injector is mainly used as a purge gas.
As described above, ALD deposition using the above vertical batch processing machine has an advantage that a production efficiency can be improved in comparison with a single-wafer processing machine. However, in a recent size-reduced semiconductor device, particularly a DRAM (Dynamic Random Access Memory), an aperture of a hole where a capacitor is to be formed has become narrower and deeper. Thus, when a film is deposited in such a deep hole, a problem of difficulty in forming an even film to the bottom of the deep hole using the gas flow sequence described in Japanese Laid-open Patent Publication No. 2004-23043 has become more prominent.
There will be described the above problem with reference to FIGS. 3 and 4. FIG. 3 is a cross-sectional view illustrating an example of a DRAM and FIG. 4 is an enlarged cross-sectional view of a capacitor.
First, a general configuration of a DRAM will be described with reference to the cross-sectional view shown in FIG. 3.
In a p-type silicon substrate 201, an n-type well 202 is formed and a first p-type well 203 is formed inside the n-type well 202. In the region other than the n-type well 202, there are formed a second p-type well 204 and an element-separating region 205. For descriptive purposes, the first p-type well 203 and the second p-type well 204 indicate a memory cell region comprising a plurality of memory cells and a peripheral circuit region, respectively.
In the first p-type well 203, there is formed a switching transistor 206 to be a word line in components in each memory cell. The switching transistor 206 comprises a drain 207, a source 208 and a gate electrode 210 via a gate insulating film 209. An interlayer insulating film 211 with a flat surface is formed such that it covers the gate electrode 210. On the source 208, a bit line 213 is formed via a bit-line contact plug 212. An interlayer insulating film 214 is formed such that it covers the bit line 213.
A capacitance-contact plug 215 is formed over a drain 207. A silicon nitride film 216 and an interlayer insulating film 217 are formed such that they covers the capacitance-contact plug 215. In given regions in the silicon nitride film 216 and the interlayer insulating film 217, holes 218 are formed for exposing the capacitance-contact plug 215. Furthermore, a lower electrode 219 of a capacitor is formed inside of the hole 218, and the lower electrode 219 of a capacitor is connected to the capacitance-contact plug 215. A dielectric body 220 and an upper electrode 221 is formed over the lower electrode 219 to provide a capacitor. An interlayer insulating film 222 is formed such that it covers the upper electrode 221. The upper electrode 221 is partly extended to a peripheral circuit region as a lead interconnection 223 and connected to an upper-layer interconnection 225 via a throughhole plug 224.
A transistor 206a comprises a drain 207, a source 208, a gate insulating film 209 and a gate electrode 210, and the transistor 206a is formed on a second p-type well 204 constituting a peripheral circuit. Interconnections 227 are formed via contact plugs 226 over the drain 207 and the source 208. A part of the interconnection 227 is connected to an upper-layer interconnection 225 via a throughhole plug 228. If necessary, a further upper interlayer insulating film and an interconnection are formed to provide a DRAM device.
FIG. 4 is an enlarged cross-sectional view of a capacitor. Here, an upper electrode is not shown. A lower electrode comprising HSGs (Hemispherical Silicon Grain) is formed inside of the hole 218 formed in the interlayer insulating film 217. Having HSGs, the lower electrode has a surface area about two times as large as that without HSGs, so that its capacitance can be approximately doubled. Although HSGs can be formed in a known manner, forming HSGs can considerably reduce a planar diameter of an in-hole space.
For example, it is assumed that a hole has a size of 3000 nm (depth) and 200 nm (the diameter indicated by D1 in the figure). After a silicon film with a thickness of 40 nm to be a matrix for HSGs is formed on the inside of the hole, HSGs are formed, so that a thickness of HSGs amounts to 80 nm. As a result, a planar diameter D2 of the remaining in-hole space is reduced to 40 nm. Considering that D2 can be 120 nm without HSGs, the remaining space has a significantly reduced diameter. For making a capacitor up, a dielectric body 220 must be formed within this narrow and deep space.
As described above, for a space in a narrow and deep hole, a gas flow sequence shown in FIG. 2 as described in Japanese Laid-open Patent Publication No. 2004-23043 has a problem of difficulty in forming a highly reliable capacity insulating film. In other words, the following steps were required for depositing a film on the bottom of a deep hole by ALD.                allowing a source gas involved in the deposition to reach the hole bottom;        purging for completely removing an excessive source gas not involved in adsorption from the hole bottom;        introducing a reactant gas to the hole bottom to react it with the adsorbed source gas completely;        purging for completely removing the reaction byproducts generated from the reaction and an excessive reactant gas from the hole bottom.        
However, the gas flow sequence described in Japanese Laid-open Patent Publication No. 2004-23043 employs only gas purging with Ar, so that gas replacement between a gas to be supplied to a hole bottom and a gas to be removed from a hole bottom is inefficient. Thus, a capacity insulating film formed on the hole bottom does not have a stoichiometric composition, resulting in an insulating film with reduced insulation ability. A reduced insulation ability of a capacity insulating film leads to increase in a leak current and insufficient charge retention properties of a capacitor, due to which a highly reliable capacitor cannot be formed.
In view of these problems, an objective of the present invention is to provide a process for depositing ALD giving a highly reliable dielectric body even in a deep hole with a small diameter, for example, a hole comprising HSGs in its inside as well as to provide a highly reliable semiconductor device by the process.